Eating artificial sweeteners may spur the very health problems that dieters try to avoid. A new multipronged study of mice and a small number of people finds that saccharin meddles with the gut’s microbial community, setting in motion metabolic changes that are associated with obesity and diabetes.

Other zero-calorie sweeteners may cause the same problems, researchers say September 17 in Nature.

Though the finding is preliminary, four of seven human volunteers eating a diet high in saccharin developed impaired glucose metabolism, a warning sign for type 2 diabetes.

“This is very interesting and scary if it really does hold for humans,” says Robert Margolskee of the Monell Chemical Senses Center in Philadelphia, who was not involved with the work. “There could be unintended consequences of these artificial sweeteners.”

Until recently, most sugar substitutes were thought to pass through the gut undigested, exerting little

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